A final report of the Outbreak Control Committee of the Surrey and Sussex Health Protection Unit describes an outbreak of E. coli O157 (VTEC O157 PT21/28) occurring in August and September 2009. This was the largest documented outbreak of VTEC O157 associated with farms in the UK. Individuals became infected either through direct or indirect contact with farm livestock. Outbreak strains were recovered from humans visiting the farm, animals housed in the petting barns, and environmental samples from the barns. It was unclear why this outbreak was so large. It was speculated that a high percentage of animals were excreting E. coli O157 and that a large number of visitors had direct contact with the animals and their environment.